Helicopters are known comprising a main rotor and a tail rotor connected to each other and rotated by one or more engines.
When powered, the main and tail rotors operate in substantially three ranges: a normal (steady) operating range, in which rotor speed (rpm) normally ranges between 96% and 102% of a given nominal speed; a lower operating range, in which rotor speed normally ranges between 90% and 96%; and an upper operating range, in which rotor speed normally ranges between 102% and 106%. These rotation speeds are obviously only indicative, in that different helicopters have different percentage operating ranges.
Automatic systems are known, as described, for example, in WO 2008/48245, for reducing the noise generated by helicopters, by reducing main and tail rotor speed. The noise generated by the rotors increases rapidly alongside an increase in speed (rpm), and automatic systems operate on the basis of parameters, such as flight altitude and speed and air temperature, to assist the pilot in adjusting rotor speed to achieve a desired low-noise level.